JP2519942B2 - Anhydrous Diluent for Propylene Oxidation to Acrolein and Acrolein Oxidation to Acrylic Acid - Google Patents

Abstract

The processes for oxidation of propylene to acrolein and the oxidation of propylene to acrylic acid in two stages with acrolein as an intermediate are improved by use of anhydrous diluent gases to reduce or replace steam in the reaction streams. In particular, the use of anhydrous diluents which raise the composite flowing heat capacity of the diluent gas mixture to at least about 6.5 calories/gram mole ( DEG C) will improve selectivity to desired products and will reduce the water load on the system.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing acrolein or acrylic acid from propylene. More particularly, this invention relates to an improved process for making acrolein or acrylic acid by catalytic vapor phase oxidation of propylene in the presence of an inert anhydrous diluent.

SUMMARY OF THE PRIOR ART Generally, propylene is oxidized to acrolein in its gas phase in the presence of a molecular oxygen-containing gas and steam at high temperature with a solid metal oxide catalyst. The acrolein produced in this reaction step can be recovered or sent to a second reactor operating in series with the first reactor to oxidize the acrolein to acrylic acid without separating it. For example, U.S. Pat. No. 4,147,885
The publication states that steam mixing is widely practiced to avoid combustion of the reactant gases and to increase selectivity to acrylic acid. U.S. Patent No. 3,
No. 475,488 is 1-60 mol, preferably 5-30 mol of water vapor per mol of propylene or propylene + acrolein.
It discloses that it is desirable to mix water vapor into the reactant reactant gas for reasons of increasing conversion and selectivity when used in molar amounts.

Other patents also state that steam is a suitable diluent. For example, U.S. Pat.No. 3,171,859 states that "addition of water vapor is essential and water vapor not only acts as a diluent, but also improves the reaction in that it significantly reduces combustion to carbon oxides." Says. Also, U.S. Pat. No. 4,267,386 discloses that an inert diluent can be added to the reaction system, but "water is in the form of steam in an amount of 0.5 to 15 mol, preferably 0.5 to 15 mol, per mol of unsaturated hydrocarbon (ie, propylene or acrolein). It is repeated by those skilled in the art that it is desirable to be present in an amount of 2 to 15 molar.

Many oxidation catalysts have been disclosed for producing acrolein in high yield by oxidizing propylene.
The main examples are molybdenum, bismuth and iron and phosphorus,
There are catalysts containing mixed oxides with tungsten or antimony. Cobalt and / or nickel and alkali metals are common promoters.

The catalysts found to be advantageous for use in the oxidation of acrolein to acrylic acid with conversions above 98% generally contain mixed metal oxides. Such catalysts typically contain molybdenum, vanadium, tungsten, chromium, copper, niobium, tantalum and antimony.

The polar objective of the teachings in the above cited references is to obtain high performance catalysts that exhibit high selectivity to acrolein and acrylic acid at high conversions of propylene. No other factors affecting the economics or improved performance of these processes have been considered in these prior art. For example, the prior art has a high effect on the process variables of high propylene concentration, how to avoid the risk of explosion, the effect of inert feedstock of the reaction process on recovery and disposal of waste, or high over a long catalyst life. No mention is made of maintaining catalytic performance. All of these are extremely important for commercial operation.

In commercial operations, it is economically important to minimize the presence of water vapor fed to the reactor, as water vapor passes through the system and becomes a burdensome wastewater burden after the product recovery process; Of course, to the inventor's knowledge, no commercial process has been successfully carried out with a steam to propylene molar ratio of less than 1.5. Furthermore, it is extremely important to minimize by-products that are difficult to separate from the useful products or have high economic penalties for disposal.
Improvements in processes that result in high catalyst performance while minimizing the use of propylene feedstock and in conditions that can promote conditions for long useful catalyst life are important for commercial operation. The prior art does not fully address these issues.

U.S. Pat. No. 4,049,577 teaches an improved catalyst composition for making acrolein. The inventor states that a recycle gas consisting of a non-condensable fraction of the product can be used instead of steam. These inventors propose the following. That is, these recycled inerts are preferred over steam as a diluent because they increase the conversion of propylene and thus obtain a high yield and reduce the water burden on the system. The use of recycled inerts is stated to be enabled by the properties of this particular catalyst composition. Nowhere in this patent is the proposal, or is it taught that anhydrous diluents are highly selective or product mixture effective, or useful for other catalysts. No relationship between various diluents and heat capacity effects on selectivity has been proposed.

U.S. Pat. No. 3,801,634 teaches the use of inert solids mixed with active catalyst in the first and second stage reactors used to produce acrolein and acrylic acid. The inventor points out that the non-condensable second stage effluent gas can be recycled to the first stage as an inert diluent gas that can be used at least partially in place of steam. There is. These inventors have not shown any relationship between inert anhydrous diluent gas and increased product selectivity.

U.S. Pat. No. 4,031,135 provides a recirculation process in which the non-condensable gas, preferably including steam, is recycled to the feedstock of the first stage reactor as well as the intermediate stage (second stage) reactor. There is. When using anhydrous diluents, no advantage is observed for the effect of these diluents on the by-product selective mixture. However, the inventor has recognized a clear improvement efficiency of acrylic acid due in part to the use of an inert diluent other than water vapor. The inventor has not proven this to be the case, nor has it shown the heat capacity effect on the selectivity of the product.

U.S. Pat. No. 4,365,087 describes recycling a dehydration residual gas containing both an inert gas and a reactive gas to increase the concentration of recovered acrylic acid. However, the inventor considers this procedure to be insufficient. This is because the composition of the residual gas changes.

U.S. Pat. No. 4,442,308 teaches the use of inert gases as diluents in the acrolein process, but specifies their use in the case of certain first stage supported acrolein catalysts. Most commercial catalysts for the oxidation of propylene to acrolein are pure (unsupported) and do not follow the designated process of this patent.
The patent also claims that 0.5-7 mol% steam is advantageous and recommended. Nowhere in this patent does the inventor teach the advantages of anhydrous diluents for product mixtures, nor does the flow heat capacity as a key variable in effectively adjusting the selectivity of the product. .

U.S. Pat. No. 4,456,006 teaches the preparation of catalysts for the reaction of propylene to acrolein. This patent shows that when a nitrogen diluent is used with the catalyst, the nitrogen diluent exhibits an improvement over the steam diluent. This patent does not teach the heat capacity effect on product selectivity, nor does it show the reduction of by-products when using anhydrous diluent.

U.S. Pat. No. 3,717,675 describes a method for recovering acrylic acid by discharging acrolein from aqueous acid, collecting and returning it to the reactor to enhance the subsequent yield of acrylic acid. This patent mentions the use of inert diluents such as carbon oxides and nitrogen, but nothing about demonstrating their importance. In fact, it states that it is necessary to add steam to the reaction to increase the selectivity.

U.S. Pat.No. 2,068,947 discloses methacrolein using an inert anhydrous diluent gas in combination with steam to produce a product having a reduced amount of condensable materials as compared to typical steam diluent methods. It teaches a method of making methacrylic acid. The inventor cannot admit the relationship between anhydrous diluent and the reduction of acetic acid, and does not mention the improvement in selectivity that results from using various anhydrous diluents.

U.S. Pat. No. 4,147,885 describes a recirculation process in which water vapor is an essential component. The purpose of this patented invention is to recycle steam to the reactor. This is contrary to the technique of the present invention. This is because it has now been found that the reduction or absence of water vapor is advantageous.

None of the prior art recognizes the use of various inert anhydrous diluents in specific proportions to affect the resulting product mixture better than any of the commonly used catalysts. Not not.

As pointed out above, the basic two-step method of oxidizing propylene through acrolein to acrylic acid is well known and has been widely described in the literature. It is also known that the hydrous overhead gas (non-condensable material) from the acrylic acid scrubber can be recycled to the first reactor stage. It is expected that this recycle of unreacted propylene and acrolein will result in an improvement in overall yield.
The use of such recirculating steam allows for the use of U.S. Pat.
It is also possible to provide supplementary means for controlling the steam content to the first stage reactor, as taught in 7,885. The method of that patent requires that the steam content of the first stage feed be between 4 and 30% by volume for all steam obtained by the recycle steam except in the starting reactant gas inclusions. However, the presence of water vapor as low as 4% is disadvantageous. This finding has not been described or confirmed in the prior art.

SUMMARY OF THE INVENTION The present invention comprises two separate but related concepts:
It includes reducing or eliminating water vapor to reduce the water burden on the process and improving selectivity to maximize the yield of the desired product. Both of these results indicate that some or all of the water vapor diluents customarily used in the prior art methods have anhydrous diluents with a range of heat capacity (optionally containing a minimum amount of water vapor). Is achieved by using.

In a preferred embodiment, a portion of the non-condensable gas from the process,
For example, a portion of the overhead distillate from the acrylic acid scrubber is recycled to the first stage reactor feed stream to replace at least some of the steam in the feed stream.

The method of the present invention can be applied not only to the associative propylene-acrolein-acrylic acid method, but also to the individual acrolein-acrylic acid method or the acrolein-acrylic acid step (leg) of the propylene-acrylic acid method. Be understood. Thus, a portion of the product stream from the first stage propylene-acrolein reactor can be sent to the acrolein recovery process, with some or all of the non-condensable overhead gas from the acrolein scrubber system as the diluent. It can be recycled to the first and / or second stage of the acrylic acid process.

DESCRIPTION OF THE INVENTION According to the present invention, anhydrous diluent gas can be used to reduce or completely replace steam in the propylene oxidation reaction, thereby efficiently producing acrolein and acrylic acid. Was discovered. (For the purposes of the present invention, a diluent is any gas which does not react when present in the reaction stage). Moreover, using anhydrous diluent instead of steam diluent,
Two major by-products, acetaldehyde and acetic acid, are significantly reduced. These reductions in by-products are especially important. Because acetaldehyde is difficult to separate from acrolein in the recovery operation, thus causing economic inconvenience in purifying the product for sale. Similarly, acetic acid and acrylic acid are difficult to separate from each other. In order to produce acrylic acid of salable quality,
Significant energy is required to remove acetic acid more completely. Furthermore, the acetic acid separation step causes a recovery loss of acrylic acid, and the waste disposal cost for processing acetic acid is high. The present invention reduces the acetic acid processing cost for both acrolein recovery and acrylic acid recovery, and the separation cost It is possible to provide a means for reducing the above-mentioned problems, to perform a good separation by using the existing equipment, thereby preventing the recovery loss of acrylic acid and providing the possibility of a higher quality refined product.

The discovery of another solution of the invention is that by increasing the heat capacity flow of the reactant gas mixture, the yield of useful products can be significantly increased. The heat capacity has a relatively high composite heat capacity (as specified here),
It is enhanced by introducing an anhydrous diluent consisting of one or more gases of relatively high molar heat capacity. Flow heat capacity is the combined heat capacity of anhydrous diluent plus the heat capacity of the reactants, ie, the combined heat capacity of the entire gas stream. However, this flow heat capacity does not change much as a result of the reaction. For, various reaction products have higher heat capacities than the heat capacities of the reactants, and some products have lower heat capacities. In general, the flow heat capacity is not expected to change by more than about 1 heat capacity unit as a result of the reaction. Thus, the combined heat capacity of the diluent is the main variable for process control.

Increasing the heat capacity of the reactant gas mixture increases the yield to acrolein and acrolein + acrylic acid, and narrows the flammable gas range, allowing for more productive operations. At the same time, the peak temperature in the catalyst bed due to the exothermic heat of reaction is lowered and the heat of reaction released is efficiently absorbed by most of the gas stream. This reduces thermal stress in the catalyst pellet structure to reduce possible carbon deposition in the catalyst pores and reduces pressure drop (because it is necessary to meet a given production level). This is due to the lower volumetric flow of the reaction gas feedstock), which should prolong the life of the catalyst.

The present invention is advantageous in recycling diluent gas and unreacted propyne back into the reactor. The resulting low stream containing product stream provides a rich source of non-condensable diluent, thus simplifying the separation of useful products. This is particularly advantageous for acrolein recovery. This is because acrolein, which is more volatile than water, can be efficiently separated from the water produced in the reaction without loss of diluent. By using a more volatile anhydrous diluent compared to acrolein, the present invention operates an acrolein recovery system with recycling of diluent back to the reactor, unreacted propylene and unrecovered acrolein. It enables further efficiency gain and cost reduction. Such systems using steam diluents are not possible when employing prior art acrolein recovery equipment and techniques. It is also possible to carry out a recycling process in which components such as acetic acid and acrylic acid and other small amounts of heavy by-products are expelled from the recycle stream. This is important as acids and heavy by-products are likely to adversely affect catalyst life, and further helps minimize recycle handling issues such as compressor corrosion.

The raw material composition of this process must be configured so that it does not form a flammable gas mixture. According to the present invention,
The starting reactant gas mixture is typically up to about 16 gmol, preferably up to about 10 gmol of propylene per liter of first stage catalyst, from about 1.1 to about 2.1 mol of molecular oxygen per mole of propylene, and a feed stream. It contains an inert diluent gas which comprises from about 40 to about 94% by volume. Desirably, the molar ratio of complex diluent to propylene is in the range of about 2 to about 32. The diluent gas typically consists of a mixture of nitrogen, carbon dioxide, methane, ethane, propane and steam, but can also include any other inert gas. Some other useful inert gases are helium, argon, hydrogen, saturated hydrocarbon gases, N ₂ O and carbon monoxide. If steam is used in the present invention, the amount of steam should be less than about 0.4 moles, preferably about 0 to about 0.3 moles per mole of propylene. The inert diluent should be sufficient to avoid flammable mixtures when combined with propylene and molecular oxygen.
Air or oxygen can be used as a molecular oxygen source. Of course, if air is used, the nitrogen content acts as a supplemental diluent.

For each diluent gas mixture, there is a relationship that can be determined experimentally and that represents a limiting composition consisting of oxygen, propylene and a diluent in which the flammable mixture is present. Most commercial applications are carried out in a "fuel rich" manner, whereby oxygen content is a limiting factor in terms of flammability.
The concentration of propylene is determined by the performance of the catalyst and factors of commercial cost effectiveness.

It is a unique advantage of the present invention that a high propylene concentration is possible because a high complex heat capacity diluent gas mixture tends to have a wider operating range due to shrinkage of the flammable gas range. It is theorized that high levels of first stage propylene feed, as high as about 30 mol% can be achieved using the process of the present invention.

The approximate range of the raw material composition is determined based on the above generalized operation restrictions. The following ranges of first stage feedstocks are particularly useful: Propylene: 0-16 g per liter of first stage catalyst.
Mol, preferably 0~10g mole; oxygen: O ₂ / C ₃ H ₆ ratio of 1.1: A 2.1, therefore, the first stage catalyst 0~33.6g moles per liter, preferably 0
~21g mol; diluent: diluent / C ₃ H ₆ ratio of 2.0 to 32, preferably 3.5 to
Twelve.

The method of the present invention is particularly advantageous in that it does not rely on any particular catalyst as much of the prior art does, but in the case of any catalyst of choice. U.S. Pat. Nos. 3,825,600; 3,649,930 and
Any molybdenum, bismuth, iron-based mixed metal oxide catalyst such as those disclosed in 4,339,355 can be used in the propylene oxidation reactor to acrolein. The second stage of propylene oxidation to acrylic acid (ie, acrolein oxidation to acrylic acid reaction) is described in US Pat. Nos. 3,775,474; 3,954,855; 3,89
Mixed metal oxide catalysts based on Mo, V as described in 3,951 and 4,339,355 can be effectively used.

The general reaction conditions are not narrowly limited and are known in the art. The first stage reaction occurs at temperatures of 250 ° C to about 450 ° C, with temperatures of about 300 ° C to about 400 ° C being preferred. The second stage reaction requires temperatures from about 200 ° C to about 450 ° C, with a preferred range of about 250 ° C to about 375 ° C.

Operating pressures of about 1 to about 4 atmospheres are typical, but improvements of this process apply for all operating pressures, whether below atmospheric pressure, atmospheric pressure or above atmospheric pressure. Although the preferred commercial mode of operation minimizes pressure, pressure is typically maintained in the range of 2-3 atm due to system pressure drop limitations.

Flow rates can range from about 0.5 to about 15 seconds contact time, while typical commercial flow rates provide about 1.5 to about 4.0 seconds contact time. Contact times of about 1.7 to about 3.0 are preferred.

As pointed out above, the selection of a suitable heat capacity for anhydrous diluent gas is limited to proper performance of the present invention. Since the diluent gas stream consists of a mixture of several individual gases, it is advantageous to have a combined heat capacity for the total stream. The term "combined heat capacity" as used herein means the sum of the product of the volume fraction of each gas in the diluent gas mixture and its heat capacity. (Heat capacity, as used herein, is the ideal gas heat capacity measured at 330 ° C. for the definition of composite heat capacity.) The composite heat capacity for the diluent gas flowing to the first stage reactor is at least about 6.5 calories. / gmol (° C) preferably at least 8 calories / gmol (° C). Below this value, the selectivity advantage of the products of the invention is minimized. Although there is no known upper limit for the combined heat capacity, it is theorized that if it is higher than about 40, there may be unrecoverable heat loss due to absorption of reaction heat into the process stream, which is economically disadvantageous. To be done. Also, the first
There is the problem of increased afterburning at the exit of the staged reactor. It is preferred to keep the combined heat capacity in the range of about 8-20, most preferably about 10-17.

The flow heat capacity of the feed gas in the second stage reactor is primarily determined by the choice of anhydrous diluent gas fed to the first stage reactor. The first stage product mixture only slightly affects the flow heat capacity of the second stage feed. The product occupies only about 10% to 20% of the total flow volume. For example, a typical operation with 7% propylene and 13% oxygen yields acrolein, acrylic acid, acetaldehyde, acetic acid and carbon oxide in addition to water. It is almost the same as the average heat capacity of the product resulting in an average heat capacity of propylene / oxygen feedstock (approximately 0.65 cal / g mol (0 ° C) or higher for product vs. reactant).

The anhydrous diluent gas of the present invention can be a single gas or a multi-component gas mixture, subject to certain criteria. Each gas must be inert to the oxidation reaction of the process and must be non-condensing and easily separable from the reaction products.

Because each plant installation has certain limitations that affect energy usage for the entire plant, be aware of the impact on plant energy balance when using certain diluents that alter current heat recovery methods. There must be. For example, a high heat capacity diluent carries much of the heat generated by the reaction, while it relies heavily on the temperature of the bath to remove and recover the heat of reaction. Furthermore, when the exhaust gas is disposed of by combustion, the heat recovery is affected by the main changes in the diluent.

Also, catalyst poisons, such as sulfur dioxide, and gases that react with undesired by-products, such as NH ₃ to form C ₄ unsaturated compounds or acrylonitrile, should be avoided.

Another advantage of the present invention is that the steam components typically contained in the first stage feedstock can be minimized or even eliminated. There is debate among those skilled in the art as to the exact function of water vapor, for example, whether water vapor is a truly inert diluent or whether water vapor is involved in the oxidation of propylene and acrolein. However, it is an accepted practice in the art that a significant concentration of water vapor is required to successfully carry out the first and second stage reactions. Contrary to this adherence in the art, the inventor's formidable finding is that water vapor is desirably minimized, but can be done altogether. This is accomplished by replacing the steam with an inert gas diluent of the given composite heat capacity of the present invention. Therefore, it was found that the water vapor content of the raw material gas can be made essentially zero. Although not preferred, the water vapor content of the source gas is about 3% by volume of the source gas.
Can reach ranks. It is preferred to keep the steam content of the feed stream below about 2% by volume, more preferably below about 1% by volume.

Although not an absolute requirement of the present invention, it is highly preferred that the inert diluent gas used is at least in part from the process an essentially anhydrous recycle stream. Preferably,
This recycle stream consists of a portion of the non-condensable overhead gas mixture from the acrolein or acrylic acid recovery xrabber train that removes water and acrylic acid from the product mixture.
In particular, the use of a light boiling anhydrous diluent allows for recycle from the acrolein recovery process. In addition to providing an advantageous system in which the current loss of acrolein separation efficiency can be recovered and the unconverted propylene reused, recycling of process gas from the acrolein recovery process is described in the prior art. It is desirable and advantageous to recycle process gas from such acrylic acid recovery processes.

In order to minimize steam retaining overhead distillates from these scrubbers, these scrubbers should be operated within the conditions shown in Table 1.

Under most operating conditions, it will be necessary to remove the purge stream, the size and location of which is determined by the particular method used. If pure oxygen is used as the oxygen source, the purge is relatively low. When air is used as the oxygen source, there is a build up of inerts, such as nitrogen, which requires significant purging and is controlled to maintain the desired combined heat capacity. By using pure oxygen (ie, oxygen that is not inflicted with a substantial concentration of inert gas), one can maximize the diluent with a higher heat capacity than that of nitrogen. This allows for the use of high heat capacity gases such as propane which may be too expensive to be used as diluents if the purge is minimized and thus the purging results in significant losses.

In the examples below, various diluents were tested in a pilot scale reactor system consisting of two single tubular reaction vessels of typical commercial reactor tube size. The first reactor tube contained a commercial catalyst consisting of molybdenum, bismuth, iron and some cocatalyst metals typical of the first stage catalyst as described above. The second reactor tube is a commercially available second reactor tube similar to that described above.
The staged catalyst was packed and connected in series with the first reactor tube. The gaseous reaction product was sampled and separated into a condensable part and a non-condensable part. A sample of each phase was measured and analyzed by a gas chromatograph. The reaction yield and the propylene conversion rate were calculated using the measured values obtained as a result. Since these sampling procedures were performed for each of the first stage product and the second stage product, both process performance for acrolein production and process performance for acrylic acid production were measured.

The experiment was set up with a statistical design, which was used for several diluents. These diluents were nitrogen, carbon dioxide, methane, propane and steam.

For additional research, a portion of the non-condensable product stream is
A set of statistically systematic changes in the combined heat capacity of the diluent gas in a series of recirculation runs that feed fresh air and propylene to the reactor with the diluent gas stream obtained by recirculating from the staged reactor. It included planning experiments.

Normative reactant concentrations of 7.0 mol% propylene, 60.2 mol% air and 32.6 mol% diluent were used in all the experimental sets designed. The diluent consisted of steam plus anhydrous diluent additive. The ratio of water vapor to anhydrous diluent additive was varied as described in the project below.

The experiment was carried out in the 2 ³ factorial planning with a central point of Tsu 4. The independent variables were the first stage temperature, space velocity and water vapor concentration of the feed. The propylene concentration (7 mol%) of the raw material and the air-to-propylene concentration ratio (8.6) of the raw material were fixed in the same manner as the system pressure and the second stage operating conditions. The main purpose of the experimental equipment is the first for various concentrations of anhydrous diluent and water vapor.
It is to show the performance of the stage (acrolein) catalyst. The experiments of the planning group are outlined below.

The term "estimated" means a diluent gas added to a mixture of air, propylene and steam. This number does not take into account the nitrogen diluent present in the air feed.

In addition to the experiments of the basic planning group, two experiments with 0% steam were conducted outside the experimental space. Composition curve experiments were carried out at 0% steam and 1600 hr ^-1 space velocity. Several additional tests were performed to demonstrate the observation of hot spot temperatures. A linear regression paradigm was formed showing the best line fit with minimal residual variance. The t-ratio for each underlying independent variable has a confidence bound for its weight of at least 95% based on standard statistical calculations. Most variables had a confidence limit of 99% when included in the model equation. In general, the equations fit the data very well.

The words "conversion", "yield", "selectivity", "space velocity"
And "contact time" are defined as follows.

* Flow rate adjusted to standard temperature and pressure (ie 0 ° C and 1 atm).

Examples The following examples are illustrative of the invention and are not intended to limit the invention in any way. In these examples, all concentrations are mol%.

Example 1 The experimental setup for the pilot plant reactor tubes was as described above. The experimental setup consisted of two identical tubular reactors each consisting of one tube packed with the appropriate catalyst as described above. The jacket surrounding each tube was filled with a heat transfer fluid that circulated to remove heat of reaction. A thermocouple and sample tap were provided along the length of each reactor at the bottom of each reactor. The gas feedstock was metered using a mass flow meter and fed to the first reactor. The first stage effluent was then passed directly into the second stage reactor. The condensable portion of the second stage effluent was recovered from the aqueous scrubber as a liquid effluent stream. Non-condensable gas is withdrawn from the top of the scrubber and, if desired, returned to the reactor to provide additional diluent gas. The system outlet pressure was controlled at 7 psig to control the system reactor feed pressure. The propylene raw material concentration is set to 7.0% and the air raw material concentration is 60.
It was set to 2%. Additional (relative to nitrogen in air feed)
The source gas diluent contained 2.6% nitrogen and 30% steam. (In addition, about 0.2
% Was present. ) Set the system outlet pressure to 7 psig,
The reactor temperature was adjusted to 320 ° C. The results are shown in Table 1.

Example 2 Example 1 was repeated, except that the additional diluent feed contained 12.6% nitrogen and 20% steam. The results are shown in Table 1.
Shown in

Example 3 Example 1 was repeated except that the diluent raw material was nitrogen at 22.6% and steam at 10%.

Example 4 Example 1 was repeated except that the diluent gas was 32.6% nitrogen and 0% steam. Adjust the temperature 9
A first stage propylene conversion of 4.5% was obtained.

Examples 5 to 8 The conditions of Examples 1 to 4 were repeated, except that methane was used instead of nitrogen in the diluent gas, and the balance was steam.

The above first eight examples of first stage catalyst performance demonstrate that as the feedstock concentration of water vapor decreases (in the order of Examples 1 to 4 and 5), the overall yield of acetaldehyde + acetic acid also decreases. ing. This reduction is then directly replaced by the second stage reaction in the overall measurement of acetic acid yield. Furthermore, a direct comparison of Examples 1-4 with Examples 5-8 reveals that the yield of acrolein + acrylic acid is significantly higher for methane diluent (higher heat capacity) compared to nitrogen diluent. Is. The yield of these is 2
It directly replaces the overall yield of acrylic acid in the stage.

Over the range of these experiments, each percentage point of steam in the feed to the first stage results in an increased yield of acetaldehyde plus acetic acid above a steam concentration of 0%. This is shown in the following relational expression.

Yield of acetaldehyde + acetic acid = 1.7535 + 0.0304 (mol% steam -20) Example 9 The experimental setup for Examples 1-8 was repeated, but without additional anhydrous diluent. That is, the raw materials are air 60.2%, propylene 7.0% and steam 32.6%.
(4.6 mol per mol of propylene). This is considered to be the conventional composition used as a comparative example in Examples 10-14. The results of Examples 9-14 are shown in Table II.

Example 10 The experimental setup used in Examples 1 to 9 was repeated.
The supply concentration of propylene was set to 7.0%, and oxygen as the molecular oxygen-containing gas was adjusted to 60.2% of the reactor raw material stream. The remaining feed consisted of a diluent composed of propane and nitrogen. The amount of nitrogen and pilopan,
The combined gas mixture was adjusted to have the same molar heat capacity as methane alone. This corresponds to 6.4% propane and 26.2% nitrogen in the reactor feed gas stream. The system outlet pressure was controlled at 7 psig and the reactor temperature was maintained at 330 ° C.

Example 11 The conditions of Example 10 were repeated, except that the diluent was configured to have the same average molar heat capacity as steam. The propane diluent concentration was 2.0% and the nitrogen diluent concentration was 30.6% of the reactant feed mixture. The results of Examples 10 and 11 are summarized in Table II.

Example 12 The conditions of Example 11 were repeated with a propane concentration of 10.8% and a nitrogen concentration of 21.8%.

Example 13 The conditions of Example 11 were repeated with a propane concentration of 23.0% and a nitrogen concentration of 9.6%.

Example 14 In this example, the flow rates of propane feed and air feed were set to the same conditions used in Examples 10-13.
In this case, the diluent volume flow rate was reduced, but the composite heat capacity of this diluent was the same as the composite heat capacity of the diluent used in Example 12. This is based on the following raw material conditions:
The results achieved by carrying out at a space velocity of 1380 hr ^-1 , propylene 8.33%, air 69.8%, nitrogen 14.6% and propane 7.26% are shown in Table II.

By comparing the acrolein + acrylic acid yields in Examples 9 to 13, it is clear that higher heat capacity leads to higher efficiency for useful products.
Furthermore, comparing Examples 10 to 13 with Example 9 shows that the absence of steam diluent dramatically reduces the yield of acetaldehyde + acetic acid. Example 14 allows the process to be carried out using a lower volumetric flow rate of diluent by using a high combined heat capacity diluent to maintain a high yield of useful product.

Over the range of these experiments, the combined heat capacity was 7-1
When in the range of 4.1, the relationship below represents an expected trend in the yield of useful product.

Acrolein yield = 1.224 (ccp) + 69.3084 Acrolein + acrylic acid yield = 0.756 (ccp) + 83.774
4 In the above equation, ccp is the composite heat capacity of the diluent as above.

Application of Recirculation Recycling of process steam is well known in the chemical processing industry and is usually practiced to improve efficiency and process economics. More specifically, recycling a product or a portion of the product stream makes it more costly to efficiently use or replenish the raw material that does not react in one go into the reactor feed stream. The raw material can be reused. The use of anhydrous diluent has a particularly advantageous effect on the recycle operability. This makes it possible to use a recycle stream which contains only a small amount of acid, thus improving the operability of the compressor. Moreover, the prior art recirculation methods require more sophisticated sampling mechanisms to reliably measure the concentration of recirculated oxygen. Oxygen control is essential to the safe operation of these recirculation methods because they are concerned about flammable gas mixtures. However, using the anhydrous stream of the present invention, oxygen can be reliably and accurately monitored, thereby improving the reliability and operability and safety of the recirculation process.

Moreover, the anhydrous diluent method provides simple and efficient recovery and recycling of acrolein in the acrolein production system. Figure 1 shows the application of recycle to the acrolein and acrylic acid processes.

Example 15 (Recycle) The product gas from the second stage reactor was condensed to remove all condensable components. Almost 30 ps of uncondensed components of this product stream containing nitrogen, carbon dioxide, carbon monoxide, oxygen and propylene are directed to the compressor
It was compressed to ig and fed to the reactor. The oxygen and propylene contents in this recycle stream were calculated and supplemental propylene and air were added to make the reactor feed stream propylene 7.0%.
% And oxygen 12.6%. The temperature of the first stage reactor was controlled so that the conversion of propylene was 95%. Similarly, the temperature of the second stage was controlled so that the acrolein conversion rate exceeded 99%. The yield of acrolein in the first stage was 78%, and that of acrylic acid in the second stage was 12.2%. The overall (both stages) yield to acrylic acid averaged 85%.

[Brief description of drawings]

1 and 2 are flow sheets showing the application of a recycle stream to the acrolein process and the acrylic acid process.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Gordon, Jean, Hark Leader, Brent Wood Road 1506, Charleston, 25314, West Virginia, United States (56) Reference JP-A-56-95144 (JP, A) JP-A-51-36415 (JP, A)

Claims (6)

(57) [Claims] 1. A process for the production of acrolein by the catalytic oxidation of propylene using one or more recycle streams and treating a feed stream containing oxygen and an inert diluent gas. As a substantially inert, substantially anhydrous diluent feed added to one or more of one or more in a molar ratio of 2.0 to 32.0 moles per mole of propylene, containing no intentionally added water. Using a substantially inert and substantially anhydrous diluent gas, the added substantially inert and substantially anhydrous gas feed is 8 to 20 calories / gram-mol (° C) complex. Improved process having a heat capacity, characterized in that the oxygen-containing stream is 1.1 to 2.1 moles of molecular oxygen per mole of propylene. 2. Predominantly acrolein is produced in the first stage reaction, acrylic acid is predominantly produced by oxidation of acrolein in the second stage reaction, and one or more recycle streams are provided in either or both stages. In a process for the production of acrolein and acrylic acid by the two-stage catalytic oxidation of propylene, wherein both stages treat a feed stream containing oxygen and an inert diluent gas. As an inert, substantially anhydrous diluent feed, 8 to 20 calories /
Has a combined heat capacity of gram-mole (° C.), does not contain any deliberately added water, and is one or more substantially inert and substantially in a molar ratio of 2.0 to 32.0 moles per mole of propylene. With anhydrous diluent gas; propylene 1
An oxygen-containing stream containing 1.1 to 2.1 moles of molecular oxygen per mole; and having a combined heat capacity of 8 to 20 calories / gram-mol (° C) as the inert gas feed to the second stage. And an improved process comprising the use of one or more substantially inert, substantially anhydrous diluent gases that do not contain any deliberately added water. 3. A method according to claim 1 or 2, characterized in that the oxygen is from a pure oxygen source. 4. A process according to claim 1 or 2 wherein the diluent gas comprises a recycle process stream from the acrolein recovery operation. 5. The method of claim 1 or 2 wherein the diluent gas comprises a recycle process stream from an acrylic acid recovery operation. 6. A method according to claim 1 or 2, characterized in that the combined heat capacity of the inert diluent gas is between 10 and 17.